[1] Stavanger Univ Hosp, Dept Obstet & Gynecol, POB 8100, N-4068 Stavanger, Norway
[2] Univ Stavanger, Fac Hlth Sci, Dept Caring & Eth, Stavanger, Norway
[3] Stavanger Univ Hosp, Dept Res, Sect Biostat, Stavanger, Norway
[4] Univ Stavanger, Fac Hlth Sci, Dept Qual & Hlth Technol, Stavanger, Norway
[5] Norwegian Chiropract Res Fdn, Et Liv I Bevegelse ELiB, Oslo, Norway